Experimental Evidence for Azeotrope Formation from Protic Ionic Liquids

Abstract

Herein, we present experimental evidence that protic ionic liquids (PILs), derived from 1 : 1 liquid mixtures of the organic superbases 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) with carboxylic acids, form azeotropic mixtures with acid/base molar fractions different from 1 : 1. The ability of the carboxylic acids to form strong hydrogen bonds with the PIL ion pair leads to an azeotropic composition richer in the acid component. The results show that the azeotropic composition is ruled by the extent of acid-base equilibrium and the relative volatility of the neutral species in the PIL medium. The PILs show marked negative deviations from Raoult's Law with the stronger superbase (DBU) leading to an azeotropic composition closer to the equimolar 1 : 1 ratio.

Limitations of Cellulose Dissolution and Fiber Spinning in the Lyocell Process Using [mTBDH][OAc] and [DBNH][OAc] Solvents

“…The mechanism of evaporation was explained by the initial dissociation (de-ionization) of the ionic compounds into neutral acid and base, [BH] (Ribeiro et al 2017). Interestingly, although the acid has a higher volatility than the base, it was found that the base is more prone to vaporization from concentrated solutions (Ribeiro et al 2018;Ahmad et al 2016). This is a result of the tendency of the acid to form hydrogen bonds with the IL in the liquid phase, protecting it from vaporization (Ribeiro et al 2018).…”

“…Interestingly, although the acid has a higher volatility than the base, it was found that the base is more prone to vaporization from concentrated solutions (Ribeiro et al 2018;Ahmad et al 2016). This is a result of the tendency of the acid to form hydrogen bonds with the IL in the liquid phase, protecting it from vaporization (Ribeiro et al 2018). Under more drastic conditions, this phenomenon proceeds until an azeotropic composition is reached, where the system exhibits the best thermal stability (lowest vapor pressure) as noted in Figure 5 b (Baird et al 2020;Ribeiro et al 2018).…”

“…This is a result of the tendency of the acid to form hydrogen bonds with the IL in the liquid phase, protecting it from vaporization (Ribeiro et al 2018). Under more drastic conditions, this phenomenon proceeds until an azeotropic composition is reached, where the system exhibits the best thermal stability (lowest vapor pressure) as noted in Figure 5 b (Baird et al 2020;Ribeiro et al 2018). Despite this, a build-up of neutral acid in the solvent system is undesired because the solvent strength is rather attributed to the presence of an equimolar ratio of anions and cations.…”

“…Despite this, a build-up of neutral acid in the solvent system is undesired because the solvent strength is rather attributed to the presence of an equimolar ratio of anions and cations. For ILs of the same anion, the extent of the azeotropic composition can be directly related to their ionicity (Ribeiro et al 2018;Ribeiro et al 2017). ILs of high degree of ionization require higher energy to dissociate and evaporate.…”

Selection and characterization of non-ideal ionic liquids mixtures to be used in CO2 capture

“…As can be seen by the 1 [Ace] are in the 1:1 (cation:anion) proportion; the remaining are in the 1:2 (cation:anion) proportion due to the formation of an azeotrope during the distillation purification step. As reported by Ribeiro et al [25], mixtures of organic superbases with carboxylic acids lead to the formation of azeotropic mixtures with an acid/ base proportion different from the initially prepared 1:1. This happens due to the ability of carboxylic acids to form strong hydrogen bonds with the protic ionic liquid ion pair leading to an azeotropic composition rich in acid.…”